Turbidity

Among the various analytical services provided by Coriolis Pharma, turbidity measurements are offered. Turbidimetry is the measurement of light transmitted through a sample solution. The transmitted light can be measured in a UV-Vis spectrophotometer at a wavelength where the ingredients, such as proteins, peptides, DNA/RNA, and formulation excipients (e.g., amino acids) do not absorb light, i.e., typically in the range of 320–800 nm. Clarity and degree of opalescence of samples is measured according to the Ph. Eur. method described under 2.2.1 of the Ph. Eur. 7th edition. Clarity values are assigned based on comparative measurements against reference formazin suspensions I-IV according to the Table below. A sample is assessed “clear” if its opalescence is equal to that of water and/or less than that of reference suspension I.

Overview of opalescent values for reference suspensions according to Ph. Eur.

Formazin suspension

Opalescent values [NTU]

Reference suspension I

3

Reference suspension II

6

Reference suspension III

18

Reference suspension IV

30

Standard of opalescence

60

Primary opalescent suspension

4000

Turbidity measurements are simple and nonspecific, but highly useful for a relative comparison of samples, e.g., for rapid screening of protein aggregation during formulation development. The method requires limited sample preparation and is non-destructive. Although turbidimetry does not provide information about size, concentration, or nature of protein aggregates or particles, the method is often used to detect relative changes in the aggregation status. Because of its high sensitivity for small species (subvisible particles) turbidity is capable of detecting the formation of particles in liquid samples very early during forced-degradation or stability studies. It should be noted that turbidity can also originate from other factors such as high protein concentration and does not necessarily reflect the presence of aggregates or particles.

Besides a NEPHLA turbidimeter operating at 860 nm and detecting at 90° angle, Coriolis has several UV-Vis instruments that can be used for turbidity measurements, as described under UV-Vis spectroscopy.